In order to measure different geometric quantities in a welding device for optical fiber ribbons such as fiber gaps, fiber diameters, offset between the exterior sides of the claddings and between the fiber cores, etc., the positions of the fibers in the xyz-coordinate system of the machine must be determined. In order to be able to make this, the ends of the fibers must be observed in at least two directions, which are well separated from each other or equivalently two pictures must be taken in two such directions. This implies that the end region of a fiber ribbon must be observed or imaged in directions, which are located obliquely to a plane passing through the end portion of the fiber ribbon. It results in its turn that the individual fiber ends have different distances to the observing person or to an optical system in a camera, so that they cannot simultaneously be observed or depicted in full sharpness, since the pictures of the fiber ends are not located in a plane perpendicular to a line from the object to the observer or to the optical axis of the optical system.